Social interaction and communication are fundamental human functions and are conserved across cultures and other mammalian species. Yet little is known about the underlying neural dynamics that drive these behaviors. Conventional functional imaging studies using fMRI and PET that investigate the neural underpinnings of spoken language have generally been limited to investigations of single subjects and non-interactive paradigms. Thus, questions of the neural mechanisms specialized for interpersonal exchanges remain understudied. The motivating hypothesis for this investigation is that interpersonal dialogue between two individuals engages a neural complex that extends beyond the functional specializations of Broca's and Wernicke's Areas known as the canonical language system and serves as a paradigm to investigate social behavior in real-time paradigms. Specifically, we hypothesize that a neural complex specialized for multi-agent interpersonal communication incorporates synchronizing information shared between the partners (transmission), using meaningful visual and auditory input cues (reception), and integrates functions that guide turn-taking behavior (regulation) between speakers. Near-infrared spectroscopy (NIRS) provides a non- invasive imaging technique that acquires the Blood Oxygen Level Dependent (BOLD) signals using absorbance spectra, and is suitable for the study of brain function in pairs of behaving subjects which is enabled by the use of portable and wearable optodes located on the surface of the head. We propose to use fNIRS and interacting pairs of subjects to compare neural responses during monologue, dialogue, and simultaneous recitation conditions, as well as face-to-face and occluded conditions that systematically limit visual and auditory input. Computational approaches will be based on coherence measures determined by wavelet analyses between homologous regions of the two interacting brains as evidence for the neural response to social engagement and of the associated underlying neural circuitry. We anticipate that dialogue tasks will activate the canonical language system more than monologue tasks, as well as additional social brain systems that receive facial information and are associated with regulatory and control functions. For example, these areas may include fusiform gyrus (interpretation of facial information) and dorsal lateral prefrontal cortex (regulation and control of neural events) respectively. Findings will provide the basis for a transceiver model of human communication including the neural underpinnings of a transmitter (speech production), a receiver (visual and auditory input), and a regulator (control of turn-taking in a conversation). The aims of this proposal are intended to provide a foundation that enables extended studies of inter-brain synchronization during social behavior and interpersonal communication in socially-healthy individuals that can be applied to social disorders in subsequent studies.